Hydraulic fluid is used in a variety of machines to produce useful work. One prevalent example is with earth moving equipment such as front-end loaders, excavators, pipe layers, graders and the like. With such machines, hydraulic cylinders are provided and are operatively connected to various work arms or other implements and moved upon opening of valves directing hydraulic fluid to the cylinder. As the hydraulic fluid is incompressible, its introduction into the cylinder necessarily moves a rod telescopingly received within the cylinder and by connecting the rod to the implement or work arm, the implement or work arm are forced to move.
In order to provide the hydraulic fluid, one or more hydraulic pumps are typically provided on the machine and driven by the engine of the machine. Such pumps can be provided in a number of different forms, with axial piston pumps being one common example. With an axial hydraulic piston pump, a central barrel or block is rotatedly driven by the motor. The barrel includes a plurality of cylinders each of which is adapted to receive a reciprocating piston. At a driven end, each of the pistons is pivotally and slidably engaged with a swashplate angularly positioned relative to the cylinder barrel. At a work end of each cylinder, a valve plate is provided having two or more kidney-shaped inlets and outlets. During the inlet phase of operation, hydraulic fluid is drawn in through the inlet of the valve plate, and into the cylinders of the rotating barrel. This drawing in or filling of the cylinders occurs as the barrel rotates, and the pistons of the barrel proximate to the inlet move from a top dead center position to bottom dead center position. The rotation of the barrel and size of the inlets are such that once the piston reaches its bottom dead center position, the cylinders rotate out of communication with the inlet of the valve plate. Further rotation of the barrel causes the cylinders, now completely filled with hydraulic fluid, to create fluid flow as the pistons move from the bottom dead center position to the top dead center position. During travel from the bottom dead center to the top dead center position, the cylinders are placed into communication with the outlet of the valve plate such that the hydraulic fluid can be delivered from the pump to provide for useful work such as the aforementioned driving of implements and work arms provided on various earth moving equipment.
While effective, and used in industry for decades, hydraulic piston pumps are not without drawbacks. As requirements placed on such work machines are steadily increased, the speed with which the hydraulic piston pumps deliver the fluid is constantly in need of improvement. Moreover, the pressures required so as to perform necessary work are also being steadily increased. However, if the speed and pressures at which the hydraulic fluid is to be delivered are constantly increased, it is important that the cylinders be filled as quickly as possible, the fluid flow be generated as quickly as possible, and the fluid be fully exhausted from the cylinders as quickly as possible. Not only must the cylinders be filled, but they should be completely filled as any voids in the cylinder or air pockets will necessarily cause cavitation in the operation of the pump and low pump efficiencies. Such cavitation results in significant vibrations affecting pump life and performance and are to be avoided. The extremely high pressures under which the pumps are operated, also require sufficient structural rigidity within the components of the pump so as to withstand such pressures.
One example of an axial piston pump is set forth in U.S. Pat. No. 5,554,007 assigned to the present assignee. While effective, such a design does not allow for the high speeds and pressures currently being sought.